Light source device

ABSTRACT

A light source device which is used as the backlight device and does not generate sound is provided. One or more flat substrates each having a light emitting element on its surface side are supported by a chassis having a conductive flat plate surface so that back surfaces of the substrates are opposed to the flat plate surface. The substrate comprises first wiring conductive thin films on a surface side of an insulation substrate, and one or more second radiating or wiring conductive thin films on a back surface side of the insulation substrate. Two terminals of the light emitting element are connected to the two adjacent first conductive thin films. A potential of at least one of the second conductive thin films is fixed to have a constant potential difference or preferably fixed to the same potential with respect to that of the flat plate surface of the chassis.

CROSS REFERENCE TO RELATED APPLICATION

This Nonprovisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2009-197277 filed in Japan on 27 Aug. 2009 theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light source device in which one ormore flat substrates provided with a light emitting element on itssurface side are supported by a chassis provided with a conductive flatplate surface in such a matter that a back surface of the substrate isopposed to the flat plate surface, and more particularly to a lightsource device which can be used as a backlight device of a transmissiveliquid crystal display or an illuminating device.

2. Description of the Related Art

A transmissive liquid crystal display is provided with the backlightdevice as a light source on a back surface of a liquid crystal panel.Conventionally, while a CCFL (Cold Cathode Fluorescent Lamp) haspredominated as the light source of the light source device used as thebacklight, an LED (Light Emitting Diode) is expected to be used as thelight source instead of the CCFL because a technology of the LED hasbeen developed. For example, Japanese Unexamined Patent ApplicationPublication No. 2003-207780 (hereinafter, referred to as Document 1)discloses a backlight device using an LED element as a light emittingelement. According to Document 1, a light source device can implementsurface irradiation over a whole screen by diffusing the desired numberof LED elements uniformly on a substrate. In addition, the surfaceirradiation can be implemented by arranging a plurality LED modules of alinear light source in which the plurality of LED elements are linearlyarranged on a substrate, as disclosed in Japanese Unexamined PatentApplication Publication No. 2008-53062. In addition, in the case wherethe plurality of LED elements are arranged on the substrate, there is amethod for arranging the LED elements in parallel like in Document 1,and a method for arranging a predetermined number of LED elements inseries as disclosed in Japanese Examined Utility Model ApplicationPublication No. 62-34468.

Therefore, the LED module in which the plurality of LED elements arelinearly arranged on a substrate and connected in series is providedwith an anode terminal 40 and a cathode terminal 41 in the vicinity ofan end side of an insulation substrate 20, to draw out anodes andcathodes at both ends of the plurality of LED elements 1 connected inseries as schematically shown in FIGS. 11A and 11B. According to thecase shown in FIGS. 11A and 11B, the anode terminal 40 and the cathodeterminal 41 are adjacently arranged on one end of a substrate 2. FIGS.11A and 11B show an LED module 50 in which eight LED elements areconnected in series, as one example. Here, FIG. 11A shows a state beforethe LED elements 1 are mounted, and FIG. 11B shows a state after the LEDelements 1 have been mounted.

In the LED module 50, nine connecting wires 31 to 39 to connect theeight LED elements 1 in series are formed on the insulation substrate20, and the connecting wires 31 to 39 are apart from each other andtheir ends are adjacently and sequentially arranged. One end of thefirst connecting wire 31 is connected to the anode terminal 40, and theother end is connected to an electrode 42 connected to an anode of theLED element 1. Each one end of the second to eighth connecting wires 32to 38 is connected to the electrode 43 connected to the cathode of theLED element 1, and each other end thereof is connected to an electrode42 connected to an anode of the LED element 1. One end of the ninthconnecting wire 39 is connected to the cathode terminal 41 and the otherend is connected to the electrode 43 connected to the cathode of the LEDelement 1. Thus, by connecting the anode and cathode of the LED elementto the electrodes 42 and 43 provided at the adjacent ends of theadjacent connecting wires, respectively, the LED module is configured asthe module in which the eight LED elements 1 are forwardly connected inseries between the anode terminal 40 and the cathode terminal 41.

FIGS. 12 and 13 are configuration views of an LED light source device inwhich the LED modules 50 shown in FIGS. 11A and 11B are arranged by 2×4in the form of a matrix and the LEDs are arranged on a plane surface.For example, as shown in FIG. 12, according to a configuration in whichthe LED modules 50 are arranged in parallel, a control substrate 51 todrive the LED elements of each LED module 50 and to control its emittedlight is provided outside, and the anode terminal 40 and the cathodeterminal 41 of each LED module 50 are connected to the control substrate51 through an external wire 52. Furthermore, as shown in FIG. 13,according to a configuration in which four groups each composed of thetwo LED modules 50 connected in series are arranged in parallel, the oneanode terminal 40 is connected to the other cathode terminal 41 betweenthe two adjacent LED modules 50, and the other anode terminal 40 and theother cathode terminal 41 are connected to the control substrate 51through the external wire 52. In either configuration, the lightemission of each LED module 50 can be controlled by the controlsubstrate 51. In addition, regarding the configuration shown in FIG. 13,since the LED modules 50 are connected in series, the number ofterminals required for controlling the control substrate 51 can bereduced.

Recently, the televisions are required to be reduced in thickness andweight. The reduction in weight can be implemented by reducing thenumber of components and reducing the size of the component in general.As a result, the substrate of the LED backlight is also reduced in areaand in weight by arranging the strip-shaped substrates in the form ofthe array as shown in FIG. 12 instead of spreading the LED elements allover the surface in the form of an array, whereby the weight reductionis implemented. Furthermore, the substrate is reduced in weight byreducing a thickness.

In addition, a simple method is employed in fixing the LED substratewith a view to reducing labor cost and because the LED substrate isreduced in size and weight. For example, instead of fixing the LEDsubstrate with screws over the whole surface, a simple method such thatboth ends of the strip-shaped substrate are fixed with rivets (allowanceis provided as compared with the fixing with screws) is used toimplement inexpensive production.

Furthermore, as for the reduction in thickness, it is required to reducea distance from the chassis of the backlight device to a liquid crystalpanel surface in the market. As a result, a gap between the chassis ofthe backlight device and the LED substrate is considerably narrowed.

Because of the above reduction in weight, the LED substrate could bedisplaced even with small force. As a matter of course, since the LEDsubstrate is fixed to the chassis, it is not largely moved, but the LEDsubstrate could be slightly moved with small force because the substrateitself is bent due to reduction in thickness to reduce its weight, andthere is an allowance gap of the small rivet.

The inventor of this application has confirmed that the LED backlightdevice generates a noise (sound) under the above circumstances. Somelevel of sound reaches as high as 30 dB although depending on the sizeof the backlight device (corresponding liquid panel screen size), whichnegatively affects the viewing and listening of the television servingas audio visual (AV) equipment. Further examination has found that thissound phenomenon is generated due to contact between the LED substrateand the chassis provided on its back surface.

By the way, when the LED elements are connected in series on the LEDsubstrate, the same current flows in the LED elements, so that anessentially great benefit is that brightness of the LED element isuniform. Meanwhile, when the LED elements are connected in parallel, theproblem is that the brightness of the LED element is not uniform unlessthe LED elements are driven separately. In addition, when the LEDelements are driven separately, a wire for driving each LED element isneeded, and the number of terminals of the drive circuit is increased,which is a problem for the LED substrate having the many LED elements.Therefore, the above problem is solved by connecting the LED elements inseries.

However, when the LED elements are connected in series, a drive voltageof the LED elements becomes high, so that a potential fluctuation of theLED substrate due to the on/off of the LED element also becomes large,which has been turned out to cause the sound to become conspicuous.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problem of thesound phenomenon of the backlight device, and it is an object of thepresent invention to provide a light source device which can be used asthe backlight device and does not generate sound.

In order to achieve the above object, the present invention provides alight source device comprising one or more flat substrates each having alight emitting element on its surface side, the substrates beingsupported by a chassis having a conductive flat plate surface so thatback surfaces of the substrates are opposed to the flat plate surface ofthe chassis, wherein each of the substrates is provided with a pluralityof first conductive thin films for wiring formed on a surface side of aninsulation substrate, and one or more second conductive thin films forradiating or wiring formed on a back surface side of the insulationsubstrate, two terminals of the light emitting element are connected tothe two adjacent first conductive thin films, respectively and apotential of at least one of the second conductive thin films is fixedto have a constant potential difference with respect to a potential ofthe flat plate surface of the chassis.

Furthermore, as for the light source device having the abovecharacteristics, it is more preferable that the potential of at leastone of the second conductive thin films is fixed to the same potentialas the potential of the flat plate surface of the chassis.

According to the light source device having the above characteristics,since a voltage applied to a capacitor formed of parallel flat platescomposed of the second conductive thin films on the back surface side ofthe substrates and the flat plate surface of the chassis is constant orzero, the coulombic force induced between the parallel flat plates isconstant and does not change, so that a change in distance between theparallel flat plates due to fluctuation in coulombic force issuppressed, and the sound due to the oscillation or contact of theparallel flat plates can be suppressed. Especially, when the potentialof the second conductive thin films on the back surface side of thesubstrates is made to conform to that of the flat plate surface of thechassis, the coulombic force becomes zero, so that the sound due to thedisposition of the parallel flat plates can be completely eliminated.

Furthermore, as for the light source device having the abovecharacteristics, it is preferable that at last one of the secondconductive thin films is a radiating conductive thin film, and notelectrically connected to the terminals of the light emitting element.

Thus, when the second conductive thin films on the back surface side ofthe substrates are electrically isolated from the first conductive thinfilms on the surface side, the potential of the second conductive thinfilms can be optionally set, so that it can be easily made to conform tothat of the flat plate surface of the chassis, and the sound phenomenoncan be easily and surely suppressed.

Furthermore, as for the light source device having the abovecharacteristics, it is preferable that the plurality of light emittingelements are mounted on the one or more substrates, and at least two ofthe plurality of light emitting elements are connected in series to forma series circuit, and brightness of the plurality of light emittingelements of the series circuit is controlled by supplying a fixedpotential from one side end of the series circuit and driving a currentflowing in the series circuit from the other side end of the seriescircuit.

Furthermore, as for the light source device having the abovecharacteristics, it is preferable that a connection terminal is providedat an end side of each of the substrates or in a vicinity of the endside, electrically connected to the first conductive thin film formed inthe vicinity of the end side, and used to electrically connect to anexternal circuit or another substrate, in which a series circuit isformed of the light emitting elements mounted on the plurality ofsubstrates by connecting the connection terminal of one substrate to theconnection terminal of another substrate to connect the plurality ofsubstrates, and brightness of the plurality of light emitting elementsof the series circuit is controlled by supplying a fixed potential fromone side end of the series circuit and driving a current flowing in theseries circuit from the other side end of the series circuit.

Thus, when the series circuit of the light emitting elements is formedto control the brightness of the light emitting elements, the brightnessof the plurality of light emitting elements can be uniformly controlledwith the small number of control circuits. However, the amount ofdisplacement between the parallel flat plates increases when thecoulombic force fluctuates because the drive voltage becomes high, andthe sound is generated conspicuously. However, since the voltage appliedto the parallel flat plates is constant or zero, it is prevented thatthe sound is generated conspicuously because the plurality of lightemitting elements are connected in series. Therefore, the presentinvention is especially effective in the configuration in which theplurality of light emitting elements are connected in series.

Furthermore, as for the light source device having the abovecharacteristics, it is preferable that a potential of the secondconductive thin films is fixed to have a predetermined potentialdifference with respect to the potential of the flat plate surface ofthe chassis in at least one of the substrates, provided with the lightemitting element having a maximum potential fluctuation at a terminal ofthe light emitting element due to the drive of the current flowing inthe series circuit.

Thus, without needing to make the second conductive thin films on theback surface side of the substrates have a constant potential differencefrom or be the same as that of the flat plate surface of the chassiswith respect to each substrate, the sound can be effectively suppressedwhen the above is performed in only the specific substrate whosepotential fluctuation in the second conductive thin films is great dueto the capacitive coupling with the potential fluctuation of the firstconductive thin films.

Furthermore, as for the light source device having the abovecharacteristics, it is preferable that the current flowing in the seriescircuit is driven by pulse width modulation.

When the current flowing in the light emitting element is driven by thepulse width modulation, the potential fluctuation induced in the secondconductive thin films increases and the coulombic force fluctuatesbecause the current is driven intermittently, so that the displacementamount of the parallel flat plates increases. As a result, the sound isgenerated conspicuously. However, since the voltage applied between theparallel flat plates becomes constant or zero, it is prevented that thesound is generated conspicuously because the plurality of light emittingelements are connected in series. Therefore, the present invention isespecially effective in the configuration in which the plurality oflight emitting elements are connected in series and driven by the pulsewidth modulation.

Furthermore, as for the light source device having the abovecharacteristics, it is preferable that the potential of the secondconductive thin films is fixed to the same potential as the potential ofthe flat plate surface of the chassis, by a conductive member to mountthe substrates on the chassis.

Furthermore, as for the light source device having the abovecharacteristics, it is preferable that the potential of the secondconductive thin films is fixed to the same potential as the potential ofthe flat plate surface of the chassis by opening a part of an insulationfilm covering the second conductive thin films provided on the backsurface side of the substrates, and bringing the second conductive thinfilms exposed from the opening part of the insulation film into contactwith the flat plate surface of the chassis directly or through aconductive connection member.

Furthermore, as for the light source device having the abovecharacteristics, it is preferable that a second connection terminal isprovided at an end side of each of the substrates or in a vicinity ofthe end side, electrically connected to the second conductive thin filmsformed in the vicinity of the end side, and used to electrically connectto an external circuit or another substrate, in which the potential ofthe second conductive thin films is fixed to the same potential as thepotential of the flat plate surface of the chassis by electricallyconnecting the second connection terminal to the flat plate surface ofthe chassis.

According to the three kinds of light source devices described above,the second conductive thin films and the flat plate surface of thechassis can be easily set to be the same potential, and the sound can besimply and easily suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a configuration of alight source device according to one embodiment of the presentinvention;

FIGS. 2A to 2C are a plan view schematically showing a configuration ona surface side of an LED substrate before LED elements are mounted, aplan view schematically showing a configuration on the surface side ofthe LED substrate after the LED elements are mounted, and a plan viewschematically showing a configuration on a back surface side of the LEDsubstrate;

FIG. 3 is a cross-sectional view schematically showing an essential partafter the LED elements are mounted on the LED substrate;

FIG. 4 is a view schematically showing a connection example in which theplurality of LED substrates are arranged in the form of an array;

FIGS. 5A and 5B are cross-sectional views schematically showing apositional relationship and a cross-sectional structure of each part ofthe LED substrate and a chassis;

FIG. 6 is a block diagram showing one configuration example of a drivecircuit of a series circuit of the LED elements shown in FIG. 4;

FIGS. 7A and 7B are voltage waveform diagrams showing a verificationexperiment result of a mechanism of generating a sound phenomenon;

FIG. 8 is a list of voltage data showing a voltage change generated in acathode terminal of each LED element of a series circuit composed of 13LED elements;

FIG. 9 is a graph of the voltage data shown in FIG. 8;

FIG. 10 is a voltage waveform diagram showing a voltage change inducedin each radiating metal thin film on the back surface side of the LEDsubstrate due to the voltage change shown in FIGS. 7A and 7B;

FIGS. 11A and 11B are plan views schematically showing one configurationexample of a conventional LED module;

FIG. 12 is a view showing a connection example in which eight LEDmodules shown in FIGS. 11A and 11B are connected in parallel; and

FIG. 13 is a view showing a connection example in which eight LEDmodules shown in FIGS. 11A and 11B are connected in parallel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of a light source device according to the presentinvention will be described with reference to the drawings. In addition,in the following drawings, a main part is emphasized and a dimensionratio of each part does not coincide with an actual dimension ratio, inorder to easily understand the description.

As shown in FIG. 1, a light source device according to this embodiment(hereinafter, referred to as the light source device simply) isconfigured in such a manner that a flat LED substrate 2 having LEDelements 1 on its surface is mounted on a chassis 3.

The LED element 1 is formed in such a manner that one or more (such astow to four) LED chips (semiconductor bare chips) are connected inseries and sealed in a translucent package, and composed of a pair of ananode terminal and a cathode terminal.

As schematically shown in FIGS. 2A to 2C and FIG. 3, the LED substrate 2includes wiring metal thin films 21 (21 <1> to 21 <7>) and 22 providedon a surface side of a strip-shaped insulation substrate 20, and oneradiating metal thin film 23 provided on a back surface side thereof,and solder resists 24 and 25 serving as insulation films to cover thewiring metal thin films 21 and 22 and the radiating metal thin film 23are provided on both surfaces of the substrate 2. FIG. 2A is a plan viewon the surface side of the LED substrate 2, in which the LED element 1and the solder resist 24 are not shown. FIG. 2B is a plan view on theback surface side of the LED substrate 2, in which the solder resist 25is not shown. FIG. 2C is a plan view on the surface side of the LEDsubstrate 2 provided with the LED elements 1, in which the solder resist24 is not shown. FIG. 3 is a cross-sectional view schematically showingan essential part of the LED substrate 2, in which the LED element 1 isconnected to the LED substrate 2. Here, the LED element 1 corresponds toa “light emitting element”, the LED substrate 2 corresponds to a“substrate”, the wiring metal thin films 21 and 22 correspond to a“first conductive thin film”, and the radiating metal thin film 23corresponds to a “second conductive thin film”.

The insulation substrate 20 is formed of woven or unwoven glass fiber ororganic fiber impregnated with a resin (such as epoxy resin or cyanateresin). The wiring metal thin films 21 and 22 and the radiating metalthin film 23 are metal thin films composed of copper or the like, andmay be plated with gold on their top surfaces according to need. Theinsulation substrate 20 has a plurality of through holes 26 (seven inFIG. 1) through which the LED substrate 2 is secured to the chassis 3with screws or rivets. Each of the wiring metal thin films 21 and 22,the radiating metal thin film 23, and the solder resists 24 and 25 isformed so as not to overlap with the through hole 26 and its vicinity.In addition, on the surface side of the LED substrate 2, an opening partis formed in the wiring metal thin film 21 and the solder resist 24, andit is used when an optical lens to diffuse emitted light of the LEDelement 1 in a lateral direction (parallel to the substrate surface) isbonded and fixed to the insulation substrate 20 although it is notshown.

The wiring metal thin films 21 <2> to 21 <6> are provided with landparts 21 a and 21 b electrically connected to the anode terminal and thecathode terminal of the LED element 1, the wiring metal thin film 21 <1>is provided with the land part 21 a electrically connected to the anodeterminal of the LED element 1, and the wiring metal thin film 21 <7> isprovided with the land part 21 b electrically connected to the cathodeterminal of the LED element 1. The solder resists 24 on the surface sidehas opening parts to expose the land parts 21 a and 21 b, and is formedso as to cover a wiring part 21 c other than the land parts 21 a and 21b.

The one land part 21 a and the other land part 21 b of the adjacentwiring metal thin films 21 are closely provided, and the anode terminalis connected to the cathode terminal in the one LED element 1, so thatthe six LED elements 1 are connected in series between the wiring metalthin film 21 <1> and 21 <7>. Here, the number of the LED elements 1mounted on the LED substrate 2 is not limited to six. In addition, thenumber of the wiring metal thin films 21 is determined according to thenumber of the LED elements 1.

The wiring metal thin film 22 extends in a longitudinal direction of theLED substrate 2, and it is not used for electrically connecting the LEDelements 1 mounted on the same substrate but used for connecting one endto the other end of the LED substrates 2. While the solder resist 25covers the radiating metal thin film 23 on the back surface side, itdoes not necessarily have to insulate between the radiating metal thinfilm 23 and the chassis 3 because the radiating metal thin film 23 isset at the same potential as that of the chassis 3 and does not serve asthe wiring film as will be described below.

In addition, in order to efficiently diffuse heat generated due to thelight emission of the LED element 1, the radiating metal thin film 23 isprovided so as to almost completely cover the back surface of theinsulation substrate 20, on the back surface side of the LED substrate2. In addition, the wiring part 21 c of the wiring metal thin film 21 isalso patterned so as to cover the surface of the insulation substrate 20to a maximum extent.

Two-terminal connectors 27 and 28 are provided at longitudinal both endsof the LED substrate 2, and one is a male connector and the other is afemale connector. One terminal of the connector 27 is connected to thewiring metal thin film 21 <1>, and the other thereof is connected to oneend of the wiring metal thin film 22, and one terminal of the connector28 is connected to the wiring metal thin film 21 <7>, and the otherthereof is connected to the other end of the wiring metal thin film 22.The plurality of LED substrates 2 arranged in the longitudinal directionof the LED substrate 2 are connected by connecting the one connector 28sequentially to the other connector 27 between the two adjacent LEDsubstrates 2 and the two terminals of the last connector 28 areshort-circuited. Thus, a series circuit is formed of all of the LEDelements 1 mounted on the sequentially connected LED substrates 2between the two terminals of the head connector 27. Therefore, the oneterminal of the head connector 27 serves as the anode terminal of theseries circuit of the LED elements 1, and the other serves as thecathode terminal thereof. The number of the LED elements 1 in each ofthe sequentially connected LED substrate 2 is not necessarily the same,and when the LED substrates 2 having different number of LED elements 1are combined, the series number of the series circuit of the LEDelements 1 can be adjusted. As schematically shown in FIG. 4, theplurality of LED substrates 2 are arranged in the longitudinal directionof the LED substrate 2 and in its perpendicular direction in the form ofan array, and the LED substrates 2 arranged in the longitudinaldirection are connected as described above, whereby the series circuits4 of the LED elements 1 are arranged in parallel.

The chassis 3 is a metal chassis in the form of a flat box to house theLED substrates 2, and composed of a rectangular back surface plate 3 aand four side plates 3 b connected to its four sides, and its frontsurface is open. When a diffuser plate or a translucent plate materialis provided on the front surface, the light source device can be used asa backlight device for a liquid crystal display or an illuminatingdevice. Here, the back surface plate 3 a of the chassis 3 corresponds toa “flat plate surface”. In addition, the chassis 3 is fixed to theground potential in this embodiment.

Next, a description will be briefly made of a method for fixing thepotential of the radiating metal thin film 23 of the LED substrate 2 tothe same potential as that of the back surface plate 3 a of the chassis3 in order to prevent a noise (sound) of the light source device whichwill be described below.

This method includes various methods because it only has to make thepotential of the radiating metal thin film 23 conform to the potentialof the back surface plate 3 a. For example, as a first method, one endof a lead is connected to the radiating metal thin film 23 by soldering,and the other end is connected to the chassis 3. As a second method, onepart (such as a part around the through hole 26 provided in theinsulation substrate 20 on the back surface side) of the solder resist25 covering the radiating metal thin film 23 is largely opened, orwithout forming the solder resist 25 all over, the LED substrate 2 issecured to the back surface plate 3 a with a screw or rivet so that theradiating metal thin film 23 is directly in contact with the backsurface plate 3 a. In addition, in the second method, it is preferableto provide a projection part projecting toward the radiating metal thinfilm 23, on the back surface plate 3 a so that the radiating metal thinfilm 23 can be easily in contact with the back surface plate 3 a. As athird method, a conductive tape such as an aluminum tape is attached onthe solder resist 25, and it is partially pressed with a center punchand the like to penetrate the solder resist 25 to be in contact with theradiating metal thin film 23, and then the LED substrate 2 is secured tothe back surface plate 3 a with the screw or rivet so that theconductive tape is directly in contact with the back surface plate 3 a.In addition, in the third method, it is preferable to provide aprojection part projecting toward the radiating metal thin film 23, onthe back surface plate 3 a to be in contact with the conductive tape sothat the conductive tape can be easily in contact with the back surfaceplate 3 a. The projection part of the back surface plate 3 a in thesecond and third methods may be formed by attaching the conductive tapesuch as the aluminum tape on the back surface plate 3 a. As a fourthmethod, three-terminal connectors 27 and 28 are provided at longitudinalboth ends of the LED substrate 2, and one terminal of each connector isconnected to the radiating metal thin film 23, so that the terminal ofthe connector 27 connected to the radiating metal thin film 23 has thesame potential as that of the back surface plate 3 a. In addition, themethod to make the potential of the radiating metal thin film 23 conformto that of the back surface plate 3 a is not limited to the above firstto fourth methods.

Next, a detailed description will be made of a mechanism of generatingthe noise (sound) of the light source device, and the reason why thesound is effectively suppressed by setting the potential of theradiating metal thin film 23 to be the same potential as that of theback surface plate 3 a, with reference to the drawings.

FIGS. 5A and 5B are cross-sectional views schematically showing apositional relationship and a cross-sectional structure of each of theLED substrate 2 and the chassis 3. FIG. 5A shows a case where theradiating metal thin film 23 is in a floating state, and FIG. 5B shows acase where the potential of the radiating metal thin film 23 is at thesame potential as that of the back surface plate 3 a. Since theinsulation substrate 20 is interposed between the wiring metal thin film21 on the surface side and the radiating metal thin film 23 on the backsurface side, it functions as a capacitor between the wiring metal thinfilm 21 and the radiating metal thin film 23.

The potential of the wiring metal thin film 21 shown in FIGS. 5A and 5Bis a potential of one node on the series circuit 4 of the LED elements 1shown in FIG. 4. FIG. 6 shows one example of a drive circuit of theseries circuit 4 of the LED elements 1 shown in FIG. 4. As shown in FIG.6, the circuit is provided outside the light source device and composedof a AC/DC converter 5 to convert a AC power to a DC power and supplythe converted power to each anode terminal 4 a of the series circuit 4,and a control circuit 6 connected to each cathode terminal 4 b of theseries circuit 4, to control brightness of each LED element by driving acurrent flowing in each series circuit 4 by PWM (pulse widthmodulation), and controlling an effective current amount flowing in theLED element 1 of the series circuit 4.

In a case where the LED element 1 is composed of three LED chipsconnected in series, when a drive current is applied to the seriescircuit 4 to light up, voltage drop of about 11 V which exceeds forwardbias Vf (about 8 V) of the three LED chips is generated in one LEDelement 1. Therefore, the voltage drop between both ends of the LEDelement 1 is about 3 V due to intermittent on/off drive by the PWM. Whenit is assumed that one series circuit 4 is composed of 13 LED elements1, a potential change for 13 LED elements 1 (about 3 V×13=about 39 V)appears in the wiring metal thin film 21 <7> connected to the cathodeterminal 4 b of the series circuit 4 and the wiring metal thin film 22.Therefore, a potential fluctuation within a range of about 3 V to about39V appears in the wiring metal thin films 21 on the series circuit 4.In addition, according to this embodiment, a small current (such as 1 mAor less) flows in the series circuit 4 even at the time of off of theon/off drive, and roughly the same voltage as the forward bias Vf isapplied to each LED element 1. At the time of on, the larger drivecurrent (such as several 10 mA) than that at the time of off is allowedto flow. Therefore, although a complete off state in which a currentdoes not flow at all is not provided at the time of off of the turn/offdrive, in the following description (including the descriptions of FIGS.8 and 9), the change in conductive state (large or small of the current)by the intermittent drive is distinguished such that the large currentstate is an on state and the small current state is off state.

As shown in FIG. 5A, when the radiating metal thin film 23 is in thefloating state, the potential fluctuation of the wiring metal thin films21 on the surface side of the LED substrate 2 is coupled to theradiating metal thin film 23 on the back surface side of the LEDsubstrate 2, due to capacitive coupling between the wiring metal thinfilm 21 and the radiating metal thin film 23, and the potential of theradiating metal thin film 23 also fluctuates in synchronization with theintermittent on/off drive by the PWM. Here, as described above, underthe circumstances that the gap between the chassis 3 of the backlightdevice and the LED substrate 2 is very small, the LED substrate 2 issecured to the chassis 3 with the screw or the rivet, and likely to bebent. Thus, when the potential of the radiating metal thin film 23opposed in parallel to the back surface plate 3 a of the chassis 3largely fluctuates with respect to the ground potential of the chassis3, in synchronization with the PWM drive as described above, it isthought that coulombic force applied between the back surface plate 3 aand the radiating metal thin film 23 changes, and a relative distancebetween the back surface plate 3 a and the radiating metal thin film 23fluctuates, so that the thinned LED substrate 2 oscillates and comesinto contact with the back surface plate 3 a repeatedly, and the soundphenomenon is generated. In addition, when the LED substrate 2oscillates, the above sound phenomenon could be generated due tofriction with the other member such as a screw or rivet to support theLED substrate 2 even when it doe not come into contact with the backsurface plate 3 a repeatedly.

Meanwhile, as shown in FIG. 5B, when the potential of the radiatingmetal thin film 23 is set to the same potential as that of the backsurface plate 3 a of the chassis 3, the potential fluctuation of thewiring metal thin film 21 on the surface side of the LED substrate 2 isprevented from appearing on the radiating metal thin film 23 on the backsurface side of the LED substrate 2 due to the capacitive coupling, andthe coulombic force itself is prevented from being applied between theback surface plate 3 a and the radiating metal thin film 23, so that thesound phenomenon caused by the change in coulombic force can beprevented.

Next, a description will be made of a verification experiment of amechanism of generating the sound phenomenon, and a result of anexperiment to confirm a sound suppression effect.

FIGS. 7A and 7B show a voltage waveform provided by observing, with asynchroscope, a voltage change induced in the radiating metal thin film23 on the back side of the LED substrate 2 provided under the conditionthat the LED substrate 2 capable of carrying five of the LED elements 1each composed of the three LED chips connected in series and the LEDsubstrate 2 capable of carrying eight of them are connected in series toprepare test substrates in which a short circuit is caused between theadjacent land parts 21 a and 21 b without carrying the LED element 1,and voltage pulses having the same voltage amplitude are commonlyapplied to the wiring metal thin films 21 and 22 on the surface side ofthe LED substrate 2, whereby the state in which the LED element 1 ismounted and driven by the PWM is simulated. FIG. 7A shows the case wherethe radiating metal thin film 23 is in the floating state, and FIG. 7Bshows the case where the potential of the radiating metal thin film 23is the same as that of the back surface plate 3 a. In either case, anupper part shows a voltage waveform A applied to the wiring metal thinfilms 21 and 22, and a lower part shows a voltage waveform B induced inthe radiating metal thin film 23. As for the voltage pulse applied tothe wiring metal thin films 21 and 22, a frequency is 480 Hz, a dutyratio is 50%, and a voltage amplification is 45 V, and the voltagewaveform B induced in the radiating metal thin film 23 is adifferentiated waveform of the voltage waveform A, and changed within arange of about −4 V to +4 V.

In addition, the voltage waveforms shown in FIG. 7A and 7B are observedunder the condition that the LED substrate 2 is supported by the chassis3, and at the same time, subjective assessment is conducted regardingwhether the noise (sound) is generated or not. As a result, when theradiating metal thin film 23 is in the floating state shown in FIG. 7A,the sound is generated, and when the radiating metal thin film 23 hasthe same potential as that of the back surface plate 3 a shown in FIG.7B, the sound is not generated. According to the above result, it hasbeen confirmed that the voltage change induced in the radiating metalthin film 23 is related to the sound, and the sound can be eliminated bysuppressing the voltage change induced in the radiating metal thin film23.

Then, under the condition that the two kinds of LED substrates 2 used inthe voltage waveform observation shown in FIGS. 7A and 7B are connectedin series, and the LED elements 1 each composed of three LED chipsconnected in series are mounted, whereby a series circuit is formed ofthe 13 LED elements 1, potential changes of the wiring metal thin films21 connected to the cathode terminals of the 13 LED elements 1 areobserved at the time of on and off of the LED elements 1 when the seriescircuit is driven by the PWM. Its result is shown in a table in FIG. 8and in a graph in FIG. 9, and a voltage change induced in the radiatingmetal thin film 23 of each of the two kinds of LED substrates 2 is shownin FIG. 10. In this experiment, 152.2 V is applied to the anode terminalof the series circuit of the 13 LED elements 1, and a current of 60 mAis driven by the PWM at a frequency of 20 Hz and at a duty ratio of 50%from the cathode terminal of this series circuit. In FIGS. 8 and 9, the13 LED elements 1 are distinguished as LED 1 to LED 13 in this orderfrom the side close to the anode terminal of the series circuit. Inaddition, the five LED elements 1 on the side close to the anodeterminal are mounted on a first LED substrate 2A, and the eight LEDelements 1 apart from the anode terminal are mounted on a second LEDsubstrate 2B, and FIG. 10 shows the voltage changes induced in theradiating metal thin films 23 on the back surface side of the first andsecond LED substrates 2A and 2B.

As shown in FIGS. 8 and 9, since a constant voltage is applied to theanode terminal of the series circuit of the 13 LED elements 1, and thecurrent is driven from the side of the cathode terminal of the seriescircuit, the voltage changes of the LED elements 1 at the time of on andoff increase with the increasing distance from the anode terminal. As aresult, as shown in FIG. 10, of the two kinds of LED substrates 2, thevoltage change induced in the radiating metal thin film 23 of the secondLED substrate 2B on the side distant from the anode terminal of theseries circuit is larger than the voltage change induced in theradiating metal thin film 23 of the first LED substrate 2A on the sideclose to the anode terminal of the series circuit. This result coincideswith the result of the subjective evaluation revealing that the soundgenerated due to the oscillations of the second LED substrate 2B islouder than that of the first LED substrate 2A.

Therefore, in the case where the plurality of LED substrates 2 areconnected in series, and the series circuit of the LED elements 1 isformed, and the potential of the radiating metal thin film 23 of any oneof the LED substrate 2 among the plurality of LED substrates 2 is madeto conform to that of the back surface plate 3 a of the chassis 3, thesound phenomenon can be most effectively suppressed when the radiatingmetal thin film 23 of the LED substrate 2 whose voltage change inducedin the radiating metal thin film 23 is largest is set to the samepotential as that of the back surface plate 3 a of the chassis 3.

Hereinafter, a description will be made of another embodiment of thedevice of the present invention.

(1) According to the above embodiment, as one example of the drivecircuit of the series circuit 4 of the LED elements 1, as shown in FIG.6, the circuit is configured such that the predetermined DC voltage issupplied from the power supply (AC/DC converter 5) to each anodeterminal 4 a of the series circuit 4, and the control circuit 6 isconnected to each cathode terminal 4 b of the series circuit 4, and thecurrent flowing in each series circuit 4 is driven by the PWM. Here,instead of the above, a circuit may be configured such that apredetermined DC voltage (such as the ground voltage) is supplied toeach cathode terminal 4 b of the series circuit 4, and the controlcircuit 6 to which a voltage is supplied from the power supply (AC/DCconverter 5) is connected to each anode terminal 4 a of the seriescircuit 4, and the current flowing in each series circuit 4 is driven bythe PWM.

Furthermore, the drive of the series circuit 4 of the LED elements 1 isnot necessarily limited to the PWM drive. In the case of pulse drive,since the driven current is turned on and off intermittently, thepotential largely changes in each part of the wiring metal thin films 21and 22 on the surface side of the LED substrate 2, so that the soundphenomenon becomes noticeable. Thus, in another drive method also, whenthe similar sound phenomenon is generated, like the above embodiment,the potential of the radiating metal thin film 23 on the back surfaceside of the LED substrate 2 is to be made to conform to that of the backsurface plate 3 a of the chassis 3 (or have a predetermined potentialdifference as will be described below). In addition, in a case wherepulse density modulation (PDM) is used as the same pulse drive, whilethe sound phenomenon similar to the PWM drive is prominently generated,the sound phenomenon can be effectively prevented by the presentinvention.

(2) While the potential of the radiating metal thin film 23 on the backsurface side of the LED substrate 2 is set to the same as that of theback surface plate 3 a of the chassis 3 in the above embodiment in orderto suppress the noise (sound) of the light source device, the abovesound can be also suppressed when the potential of the radiating metalthin film 23 is constant and a potential difference from the potentialof the back surface plate 3 a of the chassis 3 is constant, similar tothe case where the potential of the radiating metal thin film 23 is madeto conform to that of the back surface plate 3 a, because coulombicforce applied between the radiating metal thin film 23 and the backsurface plate 3 a does not change in response to the on/off of thecurrent to drive the LED element 1.

This fact means that it is only necessary to provide a wiring metal thinfilm instead of the radiating metal thin film 23 or in addition to it,on the back surface side of the LED substrate 2 and use a drive methodto keep constant the potential supplied to the wiring metal thin film.For example, the wiring metal thin film 22 on the surface side of theLED substrate 2 is provided on the back surface side, and the wiringmetal thin film 22 is connected to the anode terminal 4 a of the seriescircuit 4 of the LED elements 1, and the anode terminal and the cathodeterminal of the LED element 1 connected to the wiring metal thin film 21on the surface side are reversed, whereby a constant potential issupplied to the wiring metal thin film 22 on the back surface side.Alternatively, as shown in the another embodiment (1), the circuit maybe configured such that each anode terminal 4 a of the series circuit 4is connected to the control circuit 6 and the current flowing in eachseries circuit 4 is driven by the PWM, and the wiring metal thin film 22on the back surface side is connected to the cathode terminal 4 b of theseries circuit 4, whereby a constant potential is supplied to the wiringmetal thin film 22 on the back surface side.

(3) The material and shape of each part shown in the above embodimentsare one example and the present invention is not limited to the above.In addition, while the connectors 27 and 28 are provided at longitudinalboth ends in the LED substrate 2, an electrode (land) to solder anexternal wire (lead or the like) may be provided on the insulationsubstrate 20 instead of the connectors 27 and 28.

(4) While the LED element is provided as the light emitting element inthe above embodiments, the present invention is not limited to the LEDelement and any element may be used as long as it can emit light by acurrent.

Although the present invention has been described in terms of apreferred embodiment, it will be appreciated that various modificationsand alterations might be made by those skilled in the art withoutdeparting from the invention. The invention should therefore be measuredin terms of the claims which follow.

1. A light source device comprising one or more flat substrates eachhaving a light emitting element on its surface side, the substratesbeing supported by a chassis having a conductive flat plate surface sothat back surfaces of the substrates are opposed to the flat platesurface of the chassis, wherein each of the substrates is provided witha plurality of first conductive thin films for wiring formed on asurface side of an insulation substrate, and one or more secondconductive thin films for radiating or wiring formed on a back surfaceside of the insulation substrate, two terminals of the light emittingelement are connected to the two adjacent first conductive thin films,respectively and a potential of at least one of the second conductivethin films is fixed to have a constant potential difference with respectto a potential of the flat plate surface of the chassis.
 2. The lightsource device according to claim 1, wherein the potential of at leastone of the second conductive thin films is fixed to the same potentialas the potential of the flat plate surface of the chassis.
 3. The lightsource device according to claim 1, wherein at last one of the secondconductive thin films is a radiating conductive thin film, and notelectrically connected to the terminals of the light emitting element.4. The light source device according to claim 3, wherein a potential ofthe radiating conductive thin film is fixed to the same potential as thepotential of the flat plate surface of the chassis.
 5. The light sourcedevice according to claim 1, wherein the plurality of light emittingelements are mounted on the one or more substrates, and at least two ofthe plurality of light emitting elements are connected in series to forma series circuit, and brightness of the plurality of light emittingelements of the series circuit is controlled by supplying a fixedpotential from one side end of the series circuit and driving a currentflowing in the series circuit from the other side end of the seriescircuit.
 6. The light source device according to claim 1, comprising aconnection terminal at an end side of each of the substrates or in avicinity of the end side, electrically connected to the first conductivethin film formed in the vicinity of the end side, and used toelectrically connect to an external circuit or the other substrate,wherein a series circuit is formed of the light emitting elementsmounted on the plurality of substrates by connecting the connectionterminal of the one substrate to the connection terminal of the othersubstrate to connect the plurality of substrates, and brightness of theplurality of light emitting elements of the series circuit is controlledby supplying a fixed potential from one side end of the series circuitand driving a current flowing in the series circuit from the other sideend of the series circuit.
 7. The light source device according to claim6, wherein a potential of the second conductive thin films is fixed tohave a predetermined potential difference with respect to the potentialof the flat plate surface of the chassis in at least one of theplurality of substrates, provided with the light emitting element havinga maximum potential fluctuation at a terminal of the light emittingelement due to drive of the current flowing in the series circuit. 8.The light source device according to claim 5, wherein the currentflowing in the series circuit is driven by pulse width modulation. 9.The light source device according to claim 6, wherein the currentflowing in the series circuit is driven by pulse width modulation. 10.The light source device according to claim 1, wherein the potential ofat least one of the second conductive thin films is fixed to the samepotential as the potential of the flat plate surface of the chassis, bya conductive member to mount the substrates on the chassis.
 11. Thelight source device according to claim 1, wherein the potential of atleast one of the second conductive thin films is fixed to the samepotential as the potential of the flat plate surface of the chassis byopening a part of an insulation film covering the second conductive thinfilms provided on the back surface side of the substrates, and bringingthe second conductive thin film exposed from the opening part of theinsulation film into contact with the flat plate surface of the chassisdirectly or through a conductive connection member.
 12. The light sourcedevice according to claim 1, comprising a second connection terminalprovided at an end side of each of the substrates or in a vicinity ofthe end side, electrically connected to the second conductive thin filmsformed in the vicinity of the end side, and used to electrically connectto an external circuit or the other substrate, wherein the potential ofat least one of the second conductive thin films is fixed to the samepotential as the potential of the flat plate surface of the chassis byelectrically connecting the second connection terminal to the flat platesurface of the chassis.